U.S. patent number 6,624,432 [Application Number 09/684,188] was granted by the patent office on 2003-09-23 for emi containment apparatus.
This patent grant is currently assigned to Shielding for Electronics, Inc.. Invention is credited to John F. Gabower, Louis Sylvester Hughes.
United States Patent |
6,624,432 |
Gabower , et al. |
September 23, 2003 |
EMI containment apparatus
Abstract
An EMI and RFI shield mounting system includes a compartmented
EMI shield constructed of a vacuum metallized thermoform having
upright hollow walls separating and surrounding the compartments.
The shield conforms to the interior of a housing for electronic
equipment, with the upright walls overlying ridges formed in the
interior of the housing. A compressible gasket is placed between
the ridges of the housing and the inner reaches of the hollow walls
of the shield. The free sides of the walls of the shield may be
abutted against ground traces on a printed circuit board on which
the shield and housing are placed. The gasket urges the metallized
free edges of the walls of the shield against the ground trace of
the printed circuit board to provide electrical conductivity
between the printed circuit board and the shield. Dimples, tabs or
protruding punctures may be formed in the free sides of the walls
to ensure conductive contact with the ground trace.
Inventors: |
Gabower; John F. (Mauston,
WI), Hughes; Louis Sylvester (Incline Village, NV) |
Assignee: |
Shielding for Electronics, Inc.
(Sunnyvale, CA)
|
Family
ID: |
22568108 |
Appl.
No.: |
09/684,188 |
Filed: |
October 6, 2000 |
Current U.S.
Class: |
250/515.1;
174/384; 174/387; 250/517.1; 361/816; 361/818; 428/161;
428/164 |
Current CPC
Class: |
H04B
1/3838 (20130101); H05K 9/003 (20130101); H05K
9/0039 (20130101); H05K 9/0084 (20130101); Y10T
428/24521 (20150115); Y10T 428/24545 (20150115) |
Current International
Class: |
H04B
1/38 (20060101); H05K 9/00 (20060101); G21F
007/00 (); G21F 003/04 (); B32B 003/00 (); B32B
015/00 (); H05K 009/00 () |
Field of
Search: |
;250/515.1
;156/659.1,633,663,654,668,655 ;412/223 ;334/85 ;379/433.1
;29/841,840 ;264/164,231,232,237,299,291,293,294,306,348,517
;439/75,528,538 ;455/90,300,575,301,117,128,351,347-349 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
US 5,740,818, 4/1998, Rumbut, Jr. (withdrawn) .
Walter Hasler, "Electroplating and Vacuum Metalizing", Technical
Report, Balzers Aktiengesellschaft, Lichtenstein, published in
Galvanotechnic, 2 (1984). .
Dieter Gwinner, "Vacuum Evaporated Aluminum for Selective Shielding
of Plastic Housings", ITEM 1993 The International Jrnl of EMC,
1993. .
Kimmel & Gerke, "Chapter 7: Shielding for EMI control . . . and
how to do it right", EDN, Jan. 20, 1994 Supplement. .
"EMI Protection in Consumer Portable Products", Electronic
Packaging and Production, Mar. 1994. .
Leonard, "What's hot and what's not in EMI shielding of plastics",
Plastic Design Forum, Mar./Apr. 1993. .
Minnesota Mining & Mfg. Co., "6100 Thermoformable EMI-Shielding
Material", 1994. .
"Silver-Coated VALOX FR-1 Film Provides Shielding For Circuit
Breakers", GE Films in Action, Jun. 1994, General Electric Co.
.
"Orion.RTM. Designs Economical EMI Shielding Box Without Cutting
Corners", product brochure, Orion Industries Incorporated. .
Concise Encyclopedia of Polymer Science & Engineering, John
Wiley & Sons, 1990 ISBN 0-471-51253-2, pp. 446-447, 744-746,
1192-1195. .
Degassing of Plastic Materials/Plasticizers, L. Holland, "Vacuum
Deposition of Thin Films", Chapman & Hall Ltd., London 1966,
pp. 46-47, 52-53. .
Aluminum Evaporation (Time-Temp.) T. LaBounty, "How Do Others
Metallize?", Midwest Technical Service, "Tips", 1980 p. 19. .
"Machinery's Handbook", E. Grerg, F. Jones and A. Horton,
Industrial Press, New York, NY 1976, pp. 2299-2301. .
D. Rigney, "Vacuum Coating", pp. 387-388, 390-410. .
Leon I. Maissel & Reinhard Glang, "Handbook of Thin-Film
Technology", McGraw-Hill 1970, pp. 1-7, 8; 1-26 et seq; 1-38, 39.
.
Placon Corp., Madison, Wisconsin Sales Brochure. .
Gabower, John H. (Jack), Thermoformed Vacuum Metallized Inserts for
EMI Shielding of Electronic Devices, Consumer Electronics Show,
Flamingo Hilton and Tower, Las Vegas, Nevada. .
Adams, Frank; "Taking the Mystery Out of Metallizing"; Technical
Publishing Company, 1975, 1976. .
Midwest Tungsten Service, Tips--Vacuum Metallizing Electrical
Problems, 1986. .
Mooney, "Trend to Lower Cost Resins Will Accelerate", Plastics
World, Apr. 1995. .
"Style CBS Circuit Board Component Shielding--Design Guide 4",
product brochure, Leader Tech, Tampa, Florida. .
Product Announcement, BMI, Inc., "Modular Shielding System"
EE-Evaluation Engineering, Apr. 1995. .
Ultrasonic Welders Advance Clamshell-Sealing Process, Packaging,
Oct. 1994. .
"Methods of Controlling EMI", EMI/RFI Shielding Guide, General
Electric Plastics Co. .
Chomerics, Inc., "EMI Shielding for Commercial Electronics", 1994.
.
Gabower, Jack (John H.), "Thermoformed Vacuum--Metallized Inserts
for EMI Shielding", Item 1995--The International Journal of EMC,
Robar Industries, Inc., 1995, pp. 120, 122, 127. .
Vacuum Platers, Inc., "VAEMAT High Performance Vacuum Evaporated
Film Coatings", Product Data Sheet. .
Vacuum Platers, Inc., Advertising Folder..
|
Primary Examiner: Lee; John R.
Assistant Examiner: Souw; Bernard E.
Attorney, Agent or Firm: Harms; Allan L.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application [is converted from] claims the benefit under 35
U.S.C. .sctn.119(e) and 37 C.F.R. .sctn.1.78 of copending
provisional patent application entitled "EMI and RFI Containment
Method for Electronic Enclosures", Ser. No. 60/158,435 filed Oct.
12, 1999.[,from which this application claims priority.]The
disclosure of provisional patent application Ser. No. 60/158,435 is
hereby incorporated in its entirety.
Claims
Having described the invention, we claim:
1. An EMI shield for mounting in a rigid housing for a circuit
board, the housing having peripheral sidewalls, comprises a
thermoform formed by heating thermoformable sheet and drawing it
into a mold or onto a die, the thermoform having a vacuum deposited
metal coating thereon of a thickness of at least one micron, said
thermoform conforming to said rigid housing and fitting
conformingly between the sidewalls thereof, said thermoform having
a peripheral, outwardly extending lip thereon, said lip having a
first surface and an opposing second surface, the circuit board
having a ground trace fixed to an outer surface thereof, said first
surface of said lip abuttable to the ground trace of the circuit
board, a gasket of elastomeric material disposed between said
sidewall and said second surface, whereby said gasket urges said
first surface of said lip into touching engagement with said ground
trace.
2. A system for containment of EMI and RFI in an electronic device
having a generally rigid housing and having a circuit board
mountable within the housing comprises a polymeric thermoform
having a peripheral sidewall, said sidewall having an outwardly
extending lip thereon, said peripheral sidewall defining at least
one polygonal compartment on said thermoform, said compartment
having an open side, said thermoform having a first face and a
second face, said thermoform having a conductive metal coating on
at least the first face thereof, the open side of said at least one
compartment coincident with said first face of said thermoform,
said circuit board having a first side populated with at least one
emitting component and having a ground trace fixed thereto, said
polygonal compartment overlying said at least one emitting
component, said ground trace in registry with said lip and
touchingly engaged therewith, said housing having at least one
opentopped enclosure formed therein, said at least one enclosure
defined by upstanding ribs on said housing, said at least one
enclosure receiving said compartment of said thermoform, said lip
in registry with said upstanding ribs, an elastomeric gasket
interposed between said ribs and said second face of said
thermoform, whereby said elastomeric gasket urges said lip into
touching engagement with said ground trace.
3. A system for containment of EMI and RFI in an electronic device
having a generally rigid housing and having a circuit board
mountable within the housing comprises a polymeric form having
multiple compartments defined by hollow walls integrally formed in
said form, each of said compartments having an open side, said form
being a thermoform, said form having a first face and a second
face, each of said open sides of said compartments coincide with
said first face of said form, said form having a conductive metal
coating on all of at least said first face thereof, said circuit
board having a first side populated with a plurality of electronic
components and having a ground trace fixed thereto, said
compartments overlying at least some of said electronic components,
said ground trace in registry with said hollow walls and touchingly
engaged therewith, said housing having at least multiple opentopped
enclosures formed therein, said enclosures defined by upstanding
ribs on said housing, said enclosures receiving said compartments
of said form, said hollow walls in registry with said upstanding
ribs, an elastomeric gasket interposed between said ribs and said
second face of said form, whereby said elastomeric gasket urges
said hollow walls into touching engagement with said ground
trace.
4. The system of claim 3 wherein said metal coating is continuous
and smooth, comprising a vacuum deposited layer at least one micron
in thickness.
5. The system of claim 3 wherein said form has a peripheral
sidewall having an outwardly extending lip thereon, said enclosure
having an outer wall, said lip in registry with said outer wall of
said enclosure, said lip in registry with said ground trace, said
gasket disposed upon said outer wall of said enclosure and under
said lip, said ground trace further in registry with said lip,
whereby said gasket further urges said lip into touching engagement
with said ground trace.
6. A system for containment of EMI and RFI in an electronic device
having a generally rigid housing and having a circuit board
mountable within the housing comprises a polymeric form having a
peripheral sidewall, said sidewall having an outwardly extending
lip thereon, said form having a first face and a second face, said
form being a thermoform, said peripheral sidewall defining at least
one polygonal compartment on said form, said compartment having an
open side, said form having a conductive metal coating on at least
the first face thereof, the open side of said at least one
compartment coincident with said first face of said form, said
circuit board having a first side populated with at least one
emitting component and having a ground trace fixed thereto, said
polygonal compartment overlying said at least one emitting
component, said ground trace in registry with said lip, said
housing having at least one opentopped enclosure formed therein,
said at least one enclosure defined by upstanding ribs on said
housing, said at least one enclosure receiving said compartment of
said form, said lip in registry with said upstanding ribs, said lip
having a multiplicity of spaced apart protrusions formed
therein.
7. The EMI containment system of claim 6 wherein said protrusions
comprise dimples pressed into said lip of said form.
8. The EMI containment system of claim 6 wherein said protrusions
comprise die cut tabs formed on and extending from said lip of said
form.
9. The EMI containment system of claim 6 wherein said protrusions
comprise die cut punctures formed on and extending from said lip of
said form.
10. The containment system of claim 6 wherein said protrusions
comprise dimples pressed into said lip of said form.
11. The containment system of claim 6 wherein said protrusions
extend from said first face of said form, whereby said protrusions
are urged by said ribs into touching engagement with said ground
trace.
12. The containment system of claim 6 wherein said protrusions
extend from said second face of said form, whereby said protrusions
urge said lip into touching engagement with said ground trace.
13. A method of shielding EMI/RFI in an electronic device, the
method comprising coupling a containment form to a printed circuit
board, said containment form being a thermoform; grounding the
containment form to a ground trace; and compressing the containment
form against the ground trace by contacting a portion of a housing
of the electronic device against the containment form.
14. The method of claim 13 wherein the containment form is a
metallized thermoform.
15. The method of claim 14 further comprising vacuum metallizing
the thermoform.
16. The method of claim 13 wherein grounding comprises contacting a
protruding lip of the containment form against the ground
trace.
17. The method of claim 13 wherein grounding comprises creating a
Faraday cage.
18. The method of claim 13 wherein compressing comprises forcing
ribs of the housing against the containment form so as to urge the
containment form against the ground trace.
19. The method of claim 18 wherein forcing comprises receiving the
ribs in cavities in the containment form.
20. The method of claim 13 further comprising positioning a
non-conductive gasket between the housing and the containment
form.
21. The method of claim 20 further comprising urging a rib of the
housing against the nonconductive gasket so as to urge the
containment form against the ground trace.
22. A system for shielding EMI/RFI, the system comprising: a
housing; a circuit board comprising a ground trace, the circuit
board being positioned within the housing; a containment form
comprising a lip which extends around a periphery of the
containment form, said containment for being a thermoform; a vacuum
metallized layer attached to the containment form, wherein the
vacuum metallized layer is capable of shielding EMI/RFI radiation;
wherein the containment form is positioned in the housing so that
the housing urges the containment form into contact with the ground
trace so as to shield the circuit board from the EMI/RFI
radiation.
23. The system of claim 22 wherein the housing comprises four side
walls and ribs, the containment form is received within the housing
between the four side walls and the ribs contact the containment
form to urge the containment form against the ground trace.
24. The system of claim 23 wherein the containment form comprises
at least one hollow wall to receive the ribs.
25. The system of claim 22 wherein the containment form comprises
dimples disposed on the lip.
26. The system of claim 22 further comprises compressing a
compressible gasket positioned between the housing and the
containment form, wherein the housing contacts the gasket to
resiliently urge the containment form against the ground trace.
27. The system of claim 23 wherein the containment form comprises a
plurality of compartments.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
This invention pertains to shielding apparatus for containing high
frequency electromagnetic radiation within a personal computer,
cellular telephone, or other electronic instrument. Electromagnetic
compatibility (EMC) is a broad term used to describe
electromagnetic interference (EMI), radio frequency interference
(RFI) and electrostatic discharge (ESD), and the above terms are
often used interchangeably.
Electronic devices are both sources and receptors of EMI which
creates a two-fold problem. Since electromagnetic radiation
penetrating the device may cause electronic failure, manufacturers
need to protect the operational integrity of their products.
Secondly, manufacturers must comply with the regulations aimed at
reducing electromagnetic radiation emitted into the atmosphere.
Proper design is necessary to prevent the device's function from
being disrupted by emissions from external sources and to minimize
its system's emissions.
The use of plastic as a housing material for electronic equipment
has contributed to problems with EMI shielding because EMI waves
pass freely through unshielded plastic without substantial
impedance or resistance. Increasing clock speeds of microprocessors
used in computing devices makes it more difficult to handle the EMI
emission faster computers generate.
Current methods for shielding of electromagnetic interference (EMI)
include the use of metal housings, metal filled polymer housings,
metal liners for housings, and conductive coatings for the interior
of rigid polymer or composite housings. Recent development in low
mass shields are shown in U.S. Pat. No. 5,811,050 to Gabower.
Shields described in that patent are commercially produced by
Shielding for Electronics, Inc. of Sunnyvale, California.
Ever increasing clock speeds of personal computers being offered
makes effective shielding more and more challenging since any gap
in an EMI shield which has one dimension in excess of one-half
wavelength may allow substantial EMI leakage, causing the unit to
fail United States Federal Communication Commission standards.
The use of metallic coatings on rigid plastic housings presents
certain manufacturing and service concerns. A slipped tool used
during assembly or a repair can cause a scratch in the metal
coating of sufficient size to cause a slot antenna, thereby making
the case totally useless, and thereby leading to a costly item
being discarded with little feasibility for successful recycling.
The seams of a metal plated plastic housing will act like slot
antennae unless the housing sections are conductively joined by the
use of overlapping joints, conductive gaskets, or conductive tape.
When the housing must be opened for a repair or retrofit, it can be
understood that some of the conductive interconnection may be
degraded by the activity of disassembly.
Further background on prior art methods and characteristics of
shielding methods may be examined in "EMI/RFI Shielding Guide"
published by the GE Plastics Division of the General Electric
Company, in "The EDN Designer's Guide to Electromagnetic
Compatibility" by Gerke & Kimmel, Supplement to EDN Magazine,
Volume 39, No. 2, (January, 1994) and in "Plastics in Electronic
Equipment Enclosures: Emerging Economic and Environmental Issues"by
Peter Mooney, published by Plastic Custom Research Services,
Adorance, N.C. (February, 1995).
SUMMARY OF THE INVENTION
The invention is a novel EMI Containment System for use in
electronic equipment enclosures. A multi-compartmented plastic form
preferably made of thermoformed plastic sheet, that is, sheet or
film material which has been heated and drawn by air pressure into
a mold or onto a die, is coated with a conductive metal coating.
The metal coating may be applied by painting or preferably by
vacuum metallization as described in U.S. Pat. No. 5,811,050 to
Gabower. The resulting metallized form has a metal coating covering
its surface to a thickness of at least one micron and the wall
thickness of the plastic form is quite small, in the range of 0.003
to 0.020 inches, resulting in an inexpensive, nestable
multi-compartment EMI shield for placement over elements mounted on
a circuit board which emit electromagnetic radiation. The
compartments of the form are arranged such that the form will
conform to the inside shape of a housing for an electronic
equipment such as a cellular phone, computer, or other device which
internally generates EMI or which is susceptible to degradation if
exposed to RFI from outside the device. The form is constructed
with a laterally extending peripheral lip and with hollow walls
which separate the compartments and which fit over interior ribs
formed within the enclosure to which the shape of the form
conforms.
Non-conductive elastomeric gasket material which may be applied as
a liquid is inserted between the tops of the ribs and outer
sidewall of the housing and under the hollow ribs and the
peripheral lip of the form, thereby providing a cushion between the
form and the housing.
A circuit board containing the EMI emitting elements is placed
against the form such that the EMI emitting elements are received
within the compartments. The circuit board is provided with a
conductive ground trace on its surface facing the form, the ground
trace circumscribing a path which coincides with the hollow ribs
and peripheral lip of the form. Contact between the metallized form
and the ground trace of the circuit board serves to enclose the EMI
emitters within a grounded enclosure to contain the EMI within the
unit and isolated from other elements within the unit. When the
circuit board is mounted within the housing, the gasket forces the
lip and hollow walls of the form into contact with the ground trace
of the circuit board.
The lip and hollow walls of the form may alternatively be formed
with closely spaced dimples, puncture protrusions, or extending
tabs, which are coated with metal and extend from the form to
increase contact between the form and the ground trace of the
circuit board, the spacing of the gap-filling dimples, puncture
protrusions or extending tabs being selected to prevent spaces
between them from acting as slot antennae. The elastomeric gasket
may be omitted when the alternate embodiment form is used.
It is an object of the invention to provide an EMI containment
apparatus which does not require. conductive gaskets to assure
creation of a grounded enclosure around EMI emitting
components.
It is another object of the invention to provide an inexpensive EMI
shield system which permits shielding of EMI emitting components of
an electronic device from susceptible components of the same
device.
It is a further object of the invention to provide an inexpensive,
lightweight EMI Shielding system.
It is a further object of the invention to provide an EMI
containment system which provides conductive contact with a ground
trace formed on the surface of a circuit board containing EMI
emitting components.
It is another object of the invention to provide an EMI shield
which may be shaped to conform to the internal ribbing of a rigid
enclosure which needs no gasket element to create contact with a
surfaced ground trace of a circuit board.
These and other objects of the invention will become apparent from
examination of the description and claims which follow.
DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a perspective view of an electronic enclosure assembly
20.
FIG. 2 is an exploded perspective view of the electronic enclosure
assembly of FIG. 1.
FIG. 3 is an exploded perspective view of a circuit board, EMI/RFI
containment form and gap-filling gasket.
FIG. 4 is a detail cross-section view along line 4--4 of FIG.
1.
FIG. 5 is a bottom plan view of the EMI/RFI mounted to a circuit
board.
FIG. 6 is a perspective view of an alternate embodiment EMI/RFI
containment form.
FIG. 7 is a detail cut-away view of the lip of one alternate
embodiment EMI containment form showing gap-filling punctures
formed on the peripheral lip of the form.
FIG. 8 is a detail cut-away view of a gap-filling tab of one
alternate embodiment EMI containment form showing gap-filling
punctures formed on the peripheral lip of the form.
FIG. 9 is a detail cut-away view of gap filling dimple of one
alternate embodiment EMI containment form showing gap-filling
punctures formed on the peripheral lip of the form.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, an electronic enclosure assembly 20 of a
cellular phone is a typical clamshell enclosure design and is shown
in the assembled configuration, as it would be used. FIG. 2 shows
an exploded view of electronic enclosure assembly 20 including a
bottom enclosure housing 10 and a top enclosure housing 12. Bottom
enclosure housing 10 contains a network of ribs 11, and a plurality
of screw bosses 14. Electronic enclosure assembly 20 is fastened
together with a plurality of screws 18, and a plurality of screw
bosses 14. This fastening method is well known in the art of
electronic enclosure design and the details have been omitted so
that the focus may be on the present invention. Electronic
enclosure assembly 20 also includes an EMI/RFI containment form
assembly 24, comprising an EMI/RFI containment form 21 coated with
a conductive coating 22, preferably aluminum applied by vacuum
metallization techniques, a printed circuit board 32, a plurality
of electronic components 36, and a liquid crystal display 44. As
shown in FIGS. 2 and 3, printed circuit board 32 is populated by a
plurality of electronic components 36 electrically connected to it,
and also has an internal ground plane 50 and an EMI/RFI ground
trace 46 that is plated and exposed, on its surface facing the form
21. The shape of EMI/RFI ground trace 46 corresponds exactly to the
shape of the top surface of EMI/RFI containment form 21, the shape
of which in turn corresponds exactly to the shape of ribs 11. Other
details of the design such as other active and passive circuit
components, speakers, buttons, switches, antennae, wires,
batteries, and corresponding holes and features in both bottom
enclosure housing 10 and top enclosure housing 12, would be
included in a functional design but have been omitted so as not to
obscure the present invention.
Referring now to FIGS. 2 and 3, EMI/RFI containment form assembly
24 comprises an EMI/RFI containment form 21, a conductive coating
22 on EM/RFI containment form 21, and a gap-filling gasket 25.
EMI/RFI containment form 21 is constructed out of either polyester
or impact modified syndiocratic polystyrene thin film sheet, with a
thickness of 0.003 inches to 0.020 inches depending on application
requirements. An example of such a material is VALOX.TM.,
manufactured by General Electric Plastics of Pittsfield, Mass., or
QUESTRA.TM., manufactured by Dow Corporation of Midland, Mich. This
sheet material is formed into the shape of EMI/RFI containment form
21 by a variety of forming processes that are well known in the
industry, such as vacuum forming, pressure forming, vacuum pressure
forming, embossing, and injection molding among others. The shape
of the compartments 23 in EMI/RFI containment form 21 are dictated
by the shape of the cavities 13 in bottom enclosure housing 10,
that is, EMI/RFI containment form 21 closely fits into the cavities
created by ribs 11 in bottom enclosure housing 10. Containment form
21 includes a peripheral lip 27 which surrounds compartment 23 and
extends laterally from outer sidewalls 29 of containment form 21.
Compartments 23 are separated by narrow hollow walls 31 which
receive ribs 11 of lower housing 10. Ribs 11 and outer wall 33 of
lower housing 10 define cavities 13. Lip 27 of containment form 21
overlies ribs 11 or outer wall 33 of lower housing when containment
assembly 24 is assembled. Gasket 25 is interposed between ribs 11
and hollow walls 31 and between lip 27 and ribs 11 on outer
sidewall 33. Conductive coating 22 is applied to EMI/RFI
containment form 21 by either a vacuum deposition or conductive
painting process that is well known in the art. Conductive coating
is preferably applied to the containment form 21 by the vacuum
metalization techniques described in U.S. Pat. No. 5,811,050 to
Gabower.
Referring now to FIGS. 5 and 6, gap-filling gasket 25 consists of
NUVA SIL.TM., a liquid elastomer material product manufactured by
Loctite Corporation. Gap-filling gasket 25 material is applied as a
liquid within the recesses of hollow walls 31 of EMI/RFI
containment form 21, and cures to an elastomeric state.
Referring now to FIGS. 2 and 4, when electronic enclosure assembly
20 is fastened together for use, EMI/RFI containment form assembly
24 is constrained by bottom enclosure housing 10 and top enclosure
housing 12. EMI/RFI containment form 21 is compressed between
printed circuit board and ribs 11. In an unassembled state,
gap-filling gasket 25 is of a thickness that is larger than the
actual distance between the top of ribs 11 and the corresponding
bottom area of EMI/RFI containment form 21. Because gap-filling
gasket 25 is a compliant elastomer, ribs 11 compresses gap-filling
gasket 25 which in turn forces EMI/RFI containment form 21 firmly
against EMI/RFI ground trace 46 on printed circuit board 32. This
firm, conductive connection between EMI/RFI containment form 21 and
EMI/RFI ground trace 46 on the printed circuit board 32 creates the
necessary contact resistance for an effective EMI/RFI shielding
seam within the given areas to be shielded in the electronic
enclosure 20. The compliance of gap-filling gasket 25 also acts to
fill tolerance gaps or slight misalignments between printed-circuit
board 32 and EMI/RFI containment form 21.
When electronic enclosure assembly 20 is powered and being used,
the flow of electricity through the electronic circuit created by
printed-circuit board 32 and electronic components 36 causes EMI or
RFI to propagate away from the device. The electromagnetic energy
is contained and prevented from propagating outside of electronic
enclosure assembly 20 by the continuous conductive enclosure
created by the combination of ground plane 50, EMI/RFI ground trace
46, and EMI/RFI containment form assembly 24, which effectively
constitutes a sealed Faraday cage. The Faraday cage is a well-known
concept in the field of electromagnetics.
Referring now to FIG. 7, an alternative embodiment shows that a
plurality of gap-filling punctures 28 may be used in place of
gap-filling gasket 25. Gap-filling punctures 28 are created by a
die-cutting process whereby a die with a plurality of discrete
blades punctures through the top surface of lip 27 and hollow walls
31 of EMI/RFI containment form 21. The die is in the exact shape of
the top-most surface of EMI/RFI containment form 21. When the
blades puncture the polyester material, they deform the material
around the puncture slightly up and away from the top surface.
Gap-filling punctures 28 are formed into EMI/RFI containment form
21 before conductive coating 22 is applied. When assembled as
described above, gap-filling punctures are forced compliantly
against EMI/RFI ground trace 46 by ribs 11 and outer wall 33. Since
gap-filling punctures 28 are covered with conductive coating 22, a
continuous, conductive shield is maintained that prohibits the
EMI/RFI that is radiated by electronic components 36 from
propagating outside of electronic enclosure assembly 24. The
spacing between punctures 28 is chosen to be less than one-half
wavelength of the EMI radiation anticipated in order to prevent
leaking of EMI.
FIG. 8 discloses a close up view of a portion of lip 27 which has
been modified with gap-filling bent tabs 52 creating upstanding
flaps closely and evenly spaced apart on lip 27 with the spaces
between neighboring gap-filling bent tabs 52 being less than
one-half wavelength of the frequency to be contained. Such
gap-filling bent tabs 52 may also be formed in hollow walls 31 of
form 21. The gap-filling bent tabs 52 are forced against ground
trace 46 by ribs 11 and outer wall 33 of housing 10 when form 21
and circuit board 32 are mounted in housing 10.
Referring now to FIG. 9, another alternative embodiment shows that
a plurality of gap-filling dimples 60 may be used in place of
gap-filling gasket 25. Gap-filling dimples 60 are created by a
forming process whereby small semi-circles are formed along the top
surface of EMI/RFI containment form 21. Gap-filling dimples 60
protrude in the direction of printed circuit board 32. Gap-filling
dimples 60 are formed into EMI/RFI containment form 21 before
conductive coating 22 is applied. When assembled as described
above, gap-filling dimples 60 are forced compliantly against
EMI/RFI ground trace 46 by ribs 11. Since gap-filling dimples are
covered with conductive coating 22, a continuous, conductive shield
is maintained that prohibits the EMI/RFI that is radiated by
electronic components 36 from propagating outside of electronic
enclosure assembly 24.
Although the description above contains many specificities, these
should not be construed as limiting the scope of the invention, but
merely providing illustration of some of the presently preferred
embodiments of this invention. EMI/RFI containment form 21 could be
manufactured out of a variety of different plastics. Gap-filling
gasket 25 could be constructed out of a variety of different
compliant materials. For example, gap-filling gasket 25 could be
die-cut out of elastomeric sheet material. Other molded-in
gap-filling features could be included other than gap-filling
dimples 60. For example, gap-filling bent tabs 52 could be molded
and die-cut into EMI/RFI containment form 21, as shown in FIG.
8.
Although the description of this invention shows a cellular phone,
this invention could also be used for RFI shielding such as may be
required in radios, portable computers, PDAs (Personal Digital
Assistants), or other devices that must be prevented from emitting
EMI.
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